This invention relates generally to the field of offshore platform structure design and construction and, in particular, to a novel method for mating a production and/or drilling deck with a self-floating substructure.
Offshore platforms have been used to expand oil and gas exploration and production to various offshore environments. As platforms have been placed in deeper water and in more hostile environments, designs have been modified to adapt to those environments, often involving larger substructure and decks. In order to reduce construction and commissioning time and costs for increasingly complex decks, the deck portion of a platform may be constructed separately from the substructure and then the two sections xe2x80x9cmatedxe2x80x9d to complete the platform construction.
Much of the current technology for mating a deck with a self-floating substructure was developed in the early 1970s with the introduction of North Sea gravity based structures (GBS). A GBS is a large structure designed to be towed to the installation location, where it is ballasted down and held in place on the sea floor by the force of gravity. Generally, North Sea GBSs support large production facilities that are not economically feasible to install by conventional lifting methods developed for steel jacket structures. The smaller production equipment for steel jackets, such as are used in the Gulf of Mexico, is often lifted into place with crane barges once the jacket is in place. In the North Sea, the large production modules and the severe wave environment made it impossible to use early conventional lifting methods for deck installations-crane barges capable of significantly heavier lifts are now available. As a consequence of this difficulty, the GBS concept evolved.
One of the main features of the GBS concept is its capacity for carrying large deck payloads during the ocean tow to the installation site. Decks used with GBSs generally consist either of production and drilling equipment modules along with a module support frame (MSF) or of an integrated deck that combines the equipment with the deck structure into an integrated unit. The deck assembly typically takes place on a pier or on purpose-built supports. Once completed, two to four special barges lift the deck off the pier or supports by deballasting the barges and then transport the deck to deeper water where the partially or wholly submerged GBS is ready for the deck transfer. Transfer to the support towers of the GBS is accomplished by deballasting the GBS and/or ballasting the barges. After the deck is transferred, the GBS is deballasted to the tow draft and prepared for the ocean tow.
The disadvantages of the North Sea GBS deck installation method are: 1) the cost associated with the use of the deck transport barges; 2) costs associated with construction of temporary piers and supports; 3) potential cost penalties associated with strengthening of the MSF or integrated deck to accommodate load reversals as a result of the deck transfer to the GBS; 4) potential construction delays when appropriate deck transport barges are not readily available; and 5) restricted weather conditions for towing multi-barge units.
In other situations, topsides facility installations have involved lifting of decks or modules onto the substructure once the substructure has been installed at the desired location. These lifts are generally performed by heavy-lift crane vessels, which, due to limited availability, can be very expensive and difficult to schedule, especially for larger decks. Following the lifts, offshore hook-up and commissioning activities may require several months for completion. Decks designed for crane-lift installation require significantly more steel in the MSF to withstand the lifting forces.
The integrated deck concept was developed to reduce the time and cost of offshore construction by building the deck as a single integrated unit and completing the majority of hook-ups prior to mating the deck with the substructure. Various techniques for such mating have been developed, including the Brown and Root Hi-deck and other float-over installation concepts. Float-over installation concepts in general involve loading the completed deck structure onto a transport barge, positioning the barge between the legs of the platform substructure, ballasting the barge down to mate the deck with the substructure, and removing the barge. Much engineering effort has gone into developing methods for reducing the load shifts caused by barge movement during the mating. A deck installed with one of the float-over methods requires less steel than is required to accommodate lifting stresses, but some extra structural steel is still required to accommodate the load shifts during both the initial loading onto a barge and the subsequent transfer of the load between the barge and the platform substructure.
While many applications of the integrated deck concept involve bringing a barge in between the legs of an offshore platform, another alternative for deck installation has been the use of catamaran and U-shaped vessels and barges to support the deck for mating with a single-column or other comparatively narrower substructure. Jacks may be used to raise the deck above the top of the substructure, allowing it to then be lowered into the desired installation position. As with the other float-over techniques, the barge is subsequently removed.
While the currently available techniques have reduced platform construction costs, there is still a need for a more cost-effective way to mate a production and/or drilling deck with a self-floating substructure.
This invention provides a method for constructing an offshore platform by combining a self-floating substructure with a self-floating deck structure. First, a self-floating offshore platform deck structure constructed with a mating surface on the bottom and a self-floating platform substructure with a mating surface on the top are obtained. In the preferred embodiment, mating is achieved by at least partially submerging the substructure, positioning the self-floating deck section above it, and then adjusting the buoyancy of either or both sections to create a bearing force between the upper and lower mating surfaces. Once mated, the substructure may be raised for towing to the installation site, at which it may be installed as either a bottom-founded or a floating platform.